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cache 分析

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/**
* $RCSfile$
* $Revision: 3144 $
* $Date: 2005-12-01 14:20:11 -0300 (Thu, 01 Dec 2005) $
*
* Copyright (C) 2004-2008 Jive Software. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jivesoftware.util.cache;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import org.jivesoftware.util.LinkedListNode;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* Default, non-distributed implementation of the Cache interface.
* The algorithm for cache is as follows: a HashMap is maintained for fast
* object lookup. Two linked lists are maintained: one keeps objects in the
* order they are accessed from cache, the other keeps objects in the order
* they were originally added to cache. When objects are added to cache, they
* are first wrapped by a CacheObject which maintains the following pieces
* of information:<ul>
*
* <li> The size of the object (in bytes).
* <li> A pointer to the node in the linked list that maintains accessed
* order for the object. Keeping a reference to the node lets us avoid
* linear scans of the linked list.
* <li> A pointer to the node in the linked list that maintains the age
* of the object in cache. Keeping a reference to the node lets us avoid
* linear scans of the linked list.</ul><p>
*
* To get an object from cache, a hash lookup is performed to get a reference
* to the CacheObject that wraps the real object we are looking for.
* The object is subsequently moved to the front of the accessed linked list
* and any necessary cache cleanups are performed. Cache deletion and expiration
* is performed as needed.
*
* @author Matt Tucker
*/
public class DefaultCache<K, V> implements Cache<K, V> {
private static final Logger Log = LoggerFactory.getLogger(DefaultCache.class);
/**
* The map the keys and values are stored in.
*/
protected Map<K, DefaultCache.CacheObject<V>> map;
/**
* Linked list to maintain order that cache objects are accessed
* in, most used to least used.
*/
protected org.jivesoftware.util.LinkedList<K> lastAccessedList;
/**
* Linked list to maintain time that cache objects were initially added
* to the cache, most recently added to oldest added.
*/
protected org.jivesoftware.util.LinkedList<K> ageList;
/**
* Maximum size in bytes that the cache can grow to.
*/
private long maxCacheSize;
/**
* Maintains the current size of the cache in bytes.
*/
private int cacheSize = 0;
/**
* Maximum length of time objects can exist in cache before expiring.
*/
protected long maxLifetime;
/**
* Maintain the number of cache hits and misses. A cache hit occurs every
* time the get method is called and the cache contains the requested
* object. A cache miss represents the opposite occurence.<p>
*
* Keeping track of cache hits and misses lets one measure how efficient
* the cache is; the higher the percentage of hits, the more efficient.
*/
protected long cacheHits, cacheMisses = 0L;
/**
* The name of the cache.
*/
private String name;
/**
* Create a new default cache and specify the maximum size of for the cache in
* bytes, and the maximum lifetime of objects.
*
* @param name a name for the cache.
* @param maxSize the maximum size of the cache in bytes. -1 means the cache
* has no max size.
* @param maxLifetime the maximum amount of time objects can exist in
* cache before being deleted. -1 means objects never expire.
*/
public DefaultCache(String name, long maxSize, long maxLifetime) {
this.name = name;
this.maxCacheSize = maxSize;
this.maxLifetime = maxLifetime;
// Our primary data structure is a HashMap. The default capacity of 11
// is too small in almost all cases, so we set it bigger.
map = new HashMap<K, CacheObject<V>>(103);
lastAccessedList = new org.jivesoftware.util.LinkedList<K>();
ageList = new org.jivesoftware.util.LinkedList<K>();
}
public synchronized V put(K key, V value) {
// Delete an old entry if it exists.
V answer = remove(key);
int objectSize = 1;
try {
objectSize = CacheSizes.sizeOfAnything(value);
}
catch (CannotCalculateSizeException e) {
Log.warn(e.getMessage(), e);
}
// If the object is bigger than the entire cache, simply don't add it.
if (maxCacheSize > 0 && objectSize > maxCacheSize * .90) {
Log.warn("Cache: " + name + " -- object with key " + key +
" is too large to fit in cache. Size is " + objectSize);
return value;
}
cacheSize += objectSize;
DefaultCache.CacheObject<V> cacheObject = new DefaultCache.CacheObject<V>(value, objectSize);
map.put(key, cacheObject);
// Make an entry into the cache order list.
LinkedListNode<K> lastAccessedNode = lastAccessedList.addFirst(key);
// Store the cache order list entry so that we can get back to it
// during later lookups.
cacheObject.lastAccessedListNode = lastAccessedNode;
// Add the object to the age list
LinkedListNode<K> ageNode = ageList.addFirst(key);
// We make an explicit call to currentTimeMillis() so that total accuracy
// of lifetime calculations is better than one second.
ageNode.timestamp = System.currentTimeMillis();
cacheObject.ageListNode = ageNode;
// If cache is too full, remove least used cache entries until it is
// not too full.
cullCache();
return answer;
}
public synchronized V get(Object key) {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
DefaultCache.CacheObject<V> cacheObject = map.get(key);
if (cacheObject == null) {
// The object didn't exist in cache, so increment cache misses.
cacheMisses++;
return null;
}
// The object exists in cache, so increment cache hits. Also, increment
// the object's read count.
cacheHits++;
cacheObject.readCount++;
// Remove the object from it's current place in the cache order list,
// and re-insert it at the front of the list.
cacheObject.lastAccessedListNode.remove();
lastAccessedList.addFirst((LinkedListNode<K>) cacheObject.lastAccessedListNode);
return cacheObject.object;
}
public synchronized V remove(Object key) {
DefaultCache.CacheObject<V> cacheObject = map.get(key);
// If the object is not in cache, stop trying to remove it.
if (cacheObject == null) {
return null;
}
// remove from the hash map
map.remove(key);
// remove from the cache order list
cacheObject.lastAccessedListNode.remove();
cacheObject.ageListNode.remove();
// remove references to linked list nodes
cacheObject.ageListNode = null;
cacheObject.lastAccessedListNode = null;
// removed the object, so subtract its size from the total.
cacheSize -= cacheObject.size;
return cacheObject.object;
}
public synchronized void clear() {
Object[] keys = map.keySet().toArray();
for (int i = 0; i < keys.length; i++) {
remove(keys[i]);
}
// Now, reset all containers.
map.clear();
lastAccessedList.clear();
lastAccessedList = new org.jivesoftware.util.LinkedList<K>();
ageList.clear();
ageList = new org.jivesoftware.util.LinkedList<K>();
cacheSize = 0;
cacheHits = 0;
cacheMisses = 0;
}
public int size() {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
return map.size();
}
public boolean isEmpty() {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
return map.isEmpty();
}
public Collection<V> values() {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
return new DefaultCache.CacheObjectCollection(map.values());
}
/**
* Wraps a cached object collection to return a view of its inner objects
*/
private final class CacheObjectCollection<V> implements Collection<V> {
private Collection<DefaultCache.CacheObject<V>> cachedObjects;
private CacheObjectCollection(Collection<DefaultCache.CacheObject<V>> cachedObjects) {
this.cachedObjects = new ArrayList<CacheObject<V>>(cachedObjects);
}
public int size() {
return cachedObjects.size();
}
public boolean isEmpty() {
return size() == 0;
}
public boolean contains(Object o) {
Iterator<V> it = iterator();
while (it.hasNext()) {
if (it.next().equals(o)) {
return true;
}
}
return false;
}
public Iterator<V> iterator() {
return new Iterator<V>() {
private final Iterator<DefaultCache.CacheObject<V>> it = cachedObjects.iterator();
public boolean hasNext() {
return it.hasNext();
}
public V next() {
if(it.hasNext()) {
DefaultCache.CacheObject<V> object = it.next();
if(object == null) {
return null;
} else {
return object.object;
}
}
else {
throw new NoSuchElementException();
}
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
public Object[] toArray() {
Object[] array = new Object[size()];
Iterator it = iterator();
int i = 0;
while (it.hasNext()) {
array[i] = it.next();
}
return array;
}
public <V>V[] toArray(V[] a) {
Iterator<V> it = (Iterator<V>) iterator();
int i = 0;
while (it.hasNext()) {
a[i++] = it.next();
}
return a;
}
public boolean containsAll(Collection<?> c) {
Iterator it = c.iterator();
while(it.hasNext()) {
if(!contains(it.next())) {
return false;
}
}
return true;
}
public boolean add(V o) {
throw new UnsupportedOperationException();
}
public boolean remove(Object o) {
throw new UnsupportedOperationException();
}
public boolean addAll(Collection<? extends V> coll) {
throw new UnsupportedOperationException();
}
public boolean removeAll(Collection<?> coll) {
throw new UnsupportedOperationException();
}
public boolean retainAll(Collection<?> coll) {
throw new UnsupportedOperationException();
}
public void clear() {
throw new UnsupportedOperationException();
}
}
public boolean containsKey(Object key) {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
return map.containsKey(key);
}
public void putAll(Map<? extends K, ? extends V> map) {
for (Iterator<? extends K> i = map.keySet().iterator(); i.hasNext();) {
K key = i.next();
V value = map.get(key);
put(key, value);
}
}
public boolean containsValue(Object value) {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
if(value == null) {
return containsNullValue();
}
Iterator it = values().iterator();
while(it.hasNext()) {
if(value.equals(it.next())) {
return true;
}
}
return false;
}
private boolean containsNullValue() {
Iterator it = values().iterator();
while(it.hasNext()) {
if(it.next() == null) {
return true;
}
}
return false;
}
public Set<Entry<K, V>> entrySet() {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
// TODO Make this work right
synchronized (this) {
final Map<K, V> result = new HashMap<K, V>();
for (final Entry<K, DefaultCache.CacheObject<V>> entry : map.entrySet()) {
result.put(entry.getKey(), entry.getValue().object);
}
return result.entrySet();
}
}
public Set<K> keySet() {
// First, clear all entries that have been in cache longer than the
// maximum defined age.
deleteExpiredEntries();
synchronized (this) {
return new HashSet<K>(map.keySet());
}
}
/**
* Returns the name of this cache. The name is completely arbitrary
* and used only for display to administrators.
*
* @return the name of this cache.
*/
public String getName() {
return name;
}
/**
* Sets the name of this cache.
*
* @param name the name of this cache.
*/
public void setName(String name) {
this.name = name;
}
/**
* Returns the number of cache hits. A cache hit occurs every
* time the get method is called and the cache contains the requested
* object.<p>
*
* Keeping track of cache hits and misses lets one measure how efficient
* the cache is; the higher the percentage of hits, the more efficient.
*
* @return the number of cache hits.
*/
public long getCacheHits() {
return cacheHits;
}
/**
* Returns the number of cache misses. A cache miss occurs every
* time the get method is called and the cache does not contain the
* requested object.<p>
*
* Keeping track of cache hits and misses lets one measure how efficient
* the cache is; the higher the percentage of hits, the more efficient.
*
* @return the number of cache hits.
*/
public long getCacheMisses() {
return cacheMisses;
}
/**
* Returns the size of the cache contents in bytes. This value is only a
* rough approximation, so cache users should expect that actual VM
* memory used by the cache could be significantly higher than the value
* reported by this method.
*
* @return the size of the cache contents in bytes.
*/
public int getCacheSize() {
return cacheSize;
}
/**
* Returns the maximum size of the cache (in bytes). If the cache grows larger
* than the max size, the least frequently used items will be removed. If
* the max cache size is set to -1, there is no size limit.
*
* @return the maximum size of the cache (-1 indicates unlimited max size).
*/
public long getMaxCacheSize() {
return maxCacheSize;
}
/**
* Sets the maximum size of the cache. If the cache grows larger
* than the max size, the least frequently used items will be removed. If
* the max cache size is set to -1, there is no size limit.
*
* @param maxCacheSize the maximum size of this cache (-1 indicates unlimited max size).
*/
public void setMaxCacheSize(int maxCacheSize) {
this.maxCacheSize = maxCacheSize;
CacheFactory.setMaxSizeProperty(name, maxCacheSize);
// It's possible that the new max size is smaller than our current cache
// size. If so, we need to delete infrequently used items.
cullCache();
}
/**
* Returns the maximum number of milleseconds that any object can live
* in cache. Once the specified number of milleseconds passes, the object
* will be automatically expried from cache. If the max lifetime is set
* to -1, then objects never expire.
*
* @return the maximum number of milleseconds before objects are expired.
*/
public long getMaxLifetime() {
return maxLifetime;
}
/**
* Sets the maximum number of milleseconds that any object can live
* in cache. Once the specified number of milleseconds passes, the object
* will be automatically expried from cache. If the max lifetime is set
* to -1, then objects never expire.
*
* @param maxLifetime the maximum number of milleseconds before objects are expired.
*/
public void setMaxLifetime(long maxLifetime) {
this.maxLifetime = maxLifetime;
CacheFactory.setMaxLifetimeProperty(name, maxLifetime);
}
/**
* Clears all entries out of cache where the entries are older than the
* maximum defined age.
*/
protected void deleteExpiredEntries() {
// Check if expiration is turned on.
if (maxLifetime <= 0) {
return;
}
// Remove all old entries. To do this, we remove objects from the end
// of the linked list until they are no longer too old. We get to avoid
// any hash lookups or looking at any more objects than is strictly
// neccessary.
LinkedListNode<K> node = ageList.getLast();
// If there are no entries in the age list, return.
if (node == null) {
return;
}
// Determine the expireTime, which is the moment in time that elements
// should expire from cache. Then, we can do an easy to check to see
// if the expire time is greater than the expire time.
long expireTime = System.currentTimeMillis() - maxLifetime;
while (expireTime > node.timestamp) {
// Remove the object
remove(node.object);
// Get the next node.
node = ageList.getLast();
// If there are no more entries in the age list, return.
if (node == null) {
return;
}
}
}
/**
* Removes objects from cache if the cache is too full. "Too full" is
* defined as within 3% of the maximum cache size. Whenever the cache is
* is too big, the least frequently used elements are deleted until the
* cache is at least 10% empty.
*/
protected final void cullCache() {
// Check if a max cache size is defined.
if (maxCacheSize < 0) {
return;
}
// See if the cache size is within 3% of being too big. If so, clean out
// cache until it's 10% free.
int desiredSize = (int)(maxCacheSize * .97);
if (cacheSize >= desiredSize) {
// First, delete any old entries to see how much memory that frees.
deleteExpiredEntries();
desiredSize = (int)(maxCacheSize * .90);
if (cacheSize > desiredSize) {
long t = System.currentTimeMillis();
do {
// Get the key and invoke the remove method on it.
remove(lastAccessedList.getLast().object);
} while (cacheSize > desiredSize);
t = System.currentTimeMillis() - t;
Log.warn("Cache " + name + " was full, shrinked to 90% in " + t + "ms.");
}
}
}
/**
* Wrapper for all objects put into cache. It's primary purpose is to maintain
* references to the linked lists that maintain the creation time of the object
* and the ordering of the most used objects.
*/
private static class CacheObject<V> {
/**
* Underlying object wrapped by the CacheObject.
*/
public V object;
/**
* The size of the Cacheable object. The size of the Cacheable
* object is only computed once when it is added to the cache. This makes
* the assumption that once objects are added to cache, they are mostly
* read-only and that their size does not change significantly over time.
*/
public int size;
/**
* A reference to the node in the cache order list. We keep the reference
* here to avoid linear scans of the list. Every time the object is
* accessed, the node is removed from its current spot in the list and
* moved to the front.
*/
public LinkedListNode<?> lastAccessedListNode;
/**
* A reference to the node in the age order list. We keep the reference
* here to avoid linear scans of the list. The reference is used if the
* object has to be deleted from the list.
*/
public LinkedListNode<?> ageListNode;
/**
* A count of the number of times the object has been read from cache.
*/
public int readCount = 0;
/**
* Creates a new cache object wrapper. The size of the Cacheable object
* must be passed in in order to prevent another possibly expensive
* lookup by querying the object itself for its size.<p>
*
* @param object the underlying Object to wrap.
* @param size the size of the Cachable object in bytes.
*/
public CacheObject(V object, int size) {
this.object = object;
this.size = size;
}
}
}原理图 实现方法 * 用 HashMap 来存储和用来做 CacheKey 查找。
* 用一个LinkedList来存储访问顺序列表
* 用一个LinkedList来存储添加时间顺序列表，即过期时间。
* HashMap 中 Key 为 CacheKey, Value 包装成一个CacheObject
* CacheObject 包含：
1) object size
2) 指向 Access List 节点的指针
3) 指向 Age List 节点的指针
其中两个List的作用
1) AccessList
当添加新元素且 List 满时，删除列表最后的元素，即最长时间没有访问的元素。
2) AgeList
当调用 get cache 时候，判断 List 末尾有无过期元素，如有向前一直删除到最后一个没有过期的元素为止。